1. Field of the Invention
This invention pertains to compositions and processes for treating fibrous substrates, such as moving paper webs during papermaking and finishing operations, and more particularly to foamable compositions containing paper treating agents used to provide treated paper.
2. Description of Background Information
The manufacture of paper, and similar flexible sheet materials, usually includes a treating step involving the application of treating agents to the preformed sheet or web as known to those in the art. Such paper treating agents, including performance or functional chemicals, can be added to assist the papermaking processing by adding process performance chemicals or can be added to the paper as product performance chemicals.
Paper treating agents applied to paper webs are usually dry solids as incorporated in the treated paper. The agents are, however, usually applied to the paper web as a fluid treating composition using a liquid vehicle to facilitate distribution, coverage and penetration of the treating agent onto the paper. Various procedures have been used to apply such fluid treating compositions during papermaking and finishing operations with limited success. Typically, the paper is passed through, or in contact with, the fluid composition thereby providing an excess of composition beyond the amount required. The excess composition is then "doctored off" using a coating thickness limiting device, such as a blade, gapped roll, air knife or the like. Alternatively, the paper is passed between squeeze rolls which, by pressure between the rolls, limits and controls the amount of applied fluid. In such processes, the reservoir or "pond" of fluid composition contacting the paper is continually recirculated using a large supply reservoir enabling consistent treatment. In addition, absorption of paper treating agents from liquid reservoirs followed by doctoring off the excess is dependent upon the moisture content of the paper web in enabling uniform absorption.
Additional limitations arise since paper, which is not precoated or presized, is usually relatively slow in wettability rates because of a "self-sizing" nature due to the presence of lignins and other natural wood materials. In typical first coating or sizing treatments, included within papermaking operations, the slow wettability of the paper is compensated for by using dilute compositions in large amounts to provide longer wetting time exposure. This is due to the general undesirability in including wetting agents for promoting paper wettability during such treating processes since wetting agents generally interfere with the desired physical properties of the treated paper and also may cause uncontrolled foaming in such liquid ponds. Such liquid composition applications therefore limit the rate at which the paper can be processed since: (1) time consuming drying operations following the treatment are required and (2) difficulty in obtaining uniform wetting of the treated paper limits high speed applications.
Additional limitations in the kinds of treating agents which can be applied by squeeze rolls arise due to various limitations in their use. Treating agent compositions must generally be low-viscosity, low-solids compositions since strong hydraulic forces generated in squeeze rolls at high speeds separate the rolls leading to loss of control in the amount of composition applied and other problems. Studies show that high-viscosity fluid penetration mechanisms govern pick-up such that a strength gradient from the paper surface to the sheet center develops generated non-uniform applications, unless saturation occurs.
Spraying paper treating agent compositions onto paper web has other limitations. Generally, only low-viscosity liquids can be utilized. Limitations in the uniformity of application of treating agent occur. In particular, uniform application of high-solid liquid treating compositions is extremely difficult to obtain since spray nozzle plugging by ingredients in the composition is a common problem. Sensitivity to spray nozzle design and performance, as well as inherent limitations in spray configurations and overlap, leads to inefficient distribution. Spraying often provides uneven deposition, particularly for wide paper webs typically used in papermaking. Spraying is also sensitive to air currents which may affect uniformity of deposition. Certain treating agents may not be used for environmental reasons due to risk of being spread through the air.
Various high-solids compositions have been developed attempting to alleviate drying restrictions enabling increased application rates. Foamed compositions have been used since volumetric expansion of the composition for obtaining uniform coverage is provided by using gas in place of liquid reducing the need for drying operations.
The application of foamed treating compositions to fibrous substrates, such as paper, is described in U.S. Pat. No. 4,263,344 (Radvan et al.), using any operative surfactant. Such foams have limited utility in applying treating compositions to the surface of paper and limited suitability for use on sized paper due to undesirable wetting or detergent action of traditional surfactants on the paper size. To overcome such difficulties, U.S. Pat. No. 4,184,914 (Jenkins) describes foamed paper treating compositions containing a hydrolyzed protein foaming agent. Such foams, however, are heat sensitive and require careful control of processing and application conditions. Furthermore, protein-based foams are not satisfactory for applying certain performance chemicals such as high-viscosity starches and cellulose derivatives to paper during papermaking operations due to the poor flow characteristics and high stability of such foams. Alternatively, U.S. Pat. No. 4,353,993 (McCrossin) describes rosin-based foamed treating compositions for surface applications to fibrous substrates.
These foam systems, however, are not fast breaking but are designed to provide foamed treating compositions used as a pond or reservoir using traditional application means, such as air-knives, rolls, brushes or the like, followed by subsequent disintegration of the foam using squeeze rolls, doctoring blades or the like. The effective use of such foam application procedures requires foamed treating compositions of sufficient stability such that when exposed to air the foam does not randomly or unevenly break down to liquid causing uneven wetting of the substrate and/or uneven doctoring by blade or squeeze rolls used in the traditional application procedures. Furthermore, recycling of foam compositions to maintain composition consistency is usually difficult and impractical where applications involving the presence of more than one paper treating agent.
Foamed treating compositions for application to fabrics is described in U.S. Pat. No. 4,099,913 (Walter et al.). Such foamed formulations are designed for use in a closed system foam applicator, such as described in U.S. Pat. No. 4,023,526 (Ashmus et al.). The described foamed treating compositions provide fast-breaking, fast-wetting, limited stability foam which reverts to liquid on contact with the substrate, providing uniform application of treating agent using high solids compositions without recycle or spillage. The application of such systems to various paper treatment is limited, however, due to interference with treated paper properties and difficulties in subsequent processing of treated paper, such as printing and certain performance finishes, due to the presence of the described foaming agents.
There is therefore a need for paper treating compositions which provide effective, uniform application for a broad selection of paper treating agents, over a wide range of composition viscosities, concentrations and the like, independent of substrate properties and application rate, which provide treated paper comparable with such treated paper, but which is substantially free of foaming agent.